Power semiconductor device for igniter

ABSTRACT

A power semiconductor device for an igniter comprises: a semiconductor switching device causing a current to flow through a primary side of an ignition coil or shutting off the current flowing through the primary side of the ignition coil; an integrated circuit driving and controlling the semiconductor switching device; and a temperature sensing element sensing temperature of the semiconductor switching device, wherein the integrated circuit including an overheat protection circuit limiting a current through the semiconductor switching device to a value lower than a current through the semiconductor switching device during normal operation, when temperature sensed by the temperature sensing element is over predetermined temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power semiconductor device for anigniter having an overheat protection function to protect asemiconductor switching device at an abnormally high temperature in anignition system for an internal combustion engine.

2. Background Art

An ignition system for an internal combustion engine such as anautomobile engine has, as components for generating a high voltage to beapplied to an ignition plug, and a power semiconductor deviceincorporating an ignition coil (inductive load), a semiconductorswitching device for driving the ignition coil and a circuit device(semiconductor integrated circuit) for controlling the semiconductorswitching device. These components constitute a so-called igniter. Theignition system also has an engine control unit (ECU) including acomputer. In many cases, an overheat protection function for protectingthe semiconductor switching device in the event of occurrence ofabnormal heat generation or the like during operation by sensing theabnormal heat generation and forcibly shutting off the current flowingthrough the semiconductor switching device is provided in the powersemiconductor device (see, for example, Japanese Patent Laid-Open No.8-338350).

Because the overheat protection function is an operation according toself-protection of the power semiconductor device, the timing ofshutting off is performed independently of ignition signal timingperformed by the ECU. There is, therefore, a possibility of ignitionoccurring at an inappropriate time in the ignition sequence as a resultof a shutoff operation by the overheat protection function to cause abackfire or knocking in the engine.

As a measure against the problem, methods have been proposed for softlyshutting off the current so as not to cause ignition at the time ofshutting off, i.e., for preventing an unnecessary ignition operation bysetting the speed of shutting off the current flowing through theprimary side coil of the ignition coil low enough to avoid inducing arcdischarge on the ignition plug (see, for example, Japanese PatentLaid-Open Nos. 2001-248529 and 2008-45514).

SUMMARY OF THE INVENTION

By the overheat protection function of the conventional powersemiconductor device for igniters, the current flowing through thesemiconductor switching device is softly shut off in the event ofincrease in temperature to an abnormally high level so as not to inducearc discharge on the ignition plug. However, the shutting operation bythe overheat protection function is started upon sensing an abnormallyhigh temperature and the shutoff state is thereafter maintained as longas the device temperature is above a predetermined constant level. Thereis, therefore, a problem that the engine is caused to enter a completelystopped state simultaneously with sensing overheat regardless of thecontrol signal on the ECU side and is maintained in the stopped state.This cannot be said to be the best measure from the viewpoint of motorvehicle fail-safe control. For the purpose of preventing an erroneousoperation, a hysteresis is ordinarily set in a comparator for overheatdetermination such that a recovery is not made unless a temperaturelower than the temperature at which the shutoff is made is againreached. Therefore a considerably long time is taken to enablerestarting of the engine.

Also, realization of a soft shutoff without inducing arc discharge inthe ignition plug requires the provision of a circuit for producing atime constant of about 10 to 100 msec. Forming such a kind of circuit inthe semiconductor integrated circuit entails a problem that the chipsize is increased or the number of manufacturing steps is increased.Forming such a partial circuit outside the semiconductor integratedcircuit also entails a problem that the manufacturing cost of the powersemiconductor device is increased due to an increase in the number ofcomponent parts.

In view of the above-described problems, an object of the presentinvention is to provide a power semiconductor device for igniters whichdoes not perform the operation to shut off the semiconductor switchingdevice at any time other than the time at which the ignition signal isissued on the ECU side, and which is, therefore, capable of preventingignition at any inappropriate time while protecting itself in the eventof increase in temperature to an abnormally high level.

According to the present invention, a power semiconductor device for anigniter comprises: a semiconductor switching device causing a current toflow through a primary side of an ignition coil or shutting off thecurrent flowing through the primary side of the ignition coil; anintegrated circuit driving and controlling the semiconductor switchingdevice; and a temperature sensing element sensing temperature of thesemiconductor switching device, wherein the integrated circuit includingan overheat protection circuit limiting a current through thesemiconductor switching device to a value lower than a current throughthe semiconductor switching device during normal operation, whentemperature sensed by the temperature sensing element is overpredetermined temperature.

In the event of increase in temperature to an abnormally high level, thecurrent through the semiconductor switching device is limited to a valuelower than its value during the normal operation to reduce Joule loss inthe semiconductor switching device, thereby protecting the semiconductorswitching device. Basically, the complete shutoff operation is notperformed at any time other than ECU ignition signal timing. Therefore,no erroneous ignition by inappropriate timing occurs and the need for asoft current shutoff circuit is eliminated. Further, the shutoffoperation is not positively performed; only limiting of the currentthrough the semiconductor switching device to a low level is performed.The engine is not stopped immediately after detection of overheat.Therefore, a time margin can be provided in which suitable steps areperformed on the ECU side.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an ignition system according to afirst embodiment of the present invention.

FIG. 2 is a timing chart for illustrating the operation of the ignitionsystem according to the first embodiment of the present invention.

FIG. 3 shows the relationships between temperature and the reversesaturation current through the Schottky barrier diode used as atemperature sensing element according to first to fourth embodiments ofthe present invention.

FIG. 4 shows the relationships between temperature and a current limitvalue applied in a semiconductor switching device according to first tofourth embodiments of the present invention.

FIG. 5 is a circuit diagram showing an ignition system according to asecond embodiment of the present invention.

FIG. 6 is a circuit diagram showing an ignition system according to athird embodiment of the present invention.

FIG. 7 is a circuit diagram showing an ignition system according to afourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 shows an embodiment of an ignition system according to thepresent invention. In the ignition system shown in FIG. 1, a powersupply Vbat such as a battery is connected to one end of a primary coil61 of an ignition coil 6, while an igniter power semiconductor device 5is connected to the other end of the primary coil 61. The power supplyVbat is also connected to one end of a secondary coil 62, and anignition plug 7 having one end grounded is connected to the other end ofthe secondary coil 62.

Furthermore, an ECU 1 outputs a control input signal for driving asemiconductor switching device 41 to the igniter power semiconductordevice.

In this ignition system, the igniter power semiconductor device 5 has asemiconductor switching device 4 including an insulated gate bipolartransistor (IGBT) 41 for causing a current to flow through the primarycoil 61 or shutting off the current flowing through the primary coil 61,and an integrated circuit 3 for driving and controlling the IGBT 41according to the control signal from the ECU 1 and other operatingconditions.

As the IGBT 41, which is a main component of the semiconductor switchingdevice 4, an IGBT having, in addition to the ordinary electrodeterminals, i.e., the collector, emitter and gate, a sense emitter forsensing the collector current Ic, through which a current proportionalto (for example, about 1/1000 of) the collector current flows, isadopted. Also, a Zener diode 42 provided for protection against a surgevoltage is connected between the collector and the gate in the reversedirection.

Further, a Schottky barrier diode 43 is provided on the same substrateas a temperature sensing element for sensing the temperature of thesemiconductor switching device 4. The anode of the Schottky barrierdiode 43 is connected to the emitter terminal of the IGBT 41, while thecathode of the Schottky barrier diode 43 is connected to the referenceside of a current mirror circuit in the integrated circuit 3 describedbelow.

The functions of the integrated circuit 3 and the ignition operation ofthe entire ignition system will now be described with reference to thetiming chart of FIG. 2.

A high-level control input signal applied at time t1 from the ECU 1 toan input terminal of the integrated circuit 3 undergoes waveform shapingin a Schmitt trigger circuit 11 and thereafter turns off a first Pch MOS12. A first current mirror circuit constituted by a second Pch MOS 17and a third Pch MOS 18 then operates to cause an output current Ig2 toflow through a first resistor 23, thereby generating a gate drivevoltage to the IGBT 41.

A reference-side current value Ig1 of the first current mirror circuitis equal to the result of subtraction of an output current value If2 ofa current-limiting circuit described below and an output current 132 ofan overheat protection circuit described below from an output currentvalue Ib1 of a constant-current source 19. With respect to thisreference-side current Ig1, a current Ig2 according to the mirror ratioof the first current mirror circuit is produced as an output current.

A collector current Ic such as shown in FIG. 2 flows through the primarycoil 61 and the IGBT 41 according to a time constant determined by theinductance and the wiring resistance of the primary coil 61.

Next, a low-level control input signal is applied at time t2 from theECU 1. The first Pch MOS 12 is thereby turned on to stop the firstcurrent mirror circuit. Charge accumulated on the gate of the IGBT 41 isdischarged in an extremely short time through the first resistor 23. Asa result, the IGBT 41 is shut off.

At this time, a high voltage of about 500 V is generated on thecollector terminal of the IGBT 41 by the primary coil 61 in thedirection to maintain the current that has been flowing. This voltage isboosted to 30 kV according to the winding ratio of the ignition coil 6to generate arc discharge on the ignition plug 7 connected to thesecondary coil 62.

Next, a case where the high-level control input signal is applied fromthe ECU 1 for a comparatively long energization time period at time t3will be described.

By the application of the high-level control input signal from the ECU1, the collector current Ic is gradually increased from time t3 in theway described above. However, a current limit value for inhibiting thecollector current In from becoming equal to or higher than apredetermined constant value is set for the purpose of preventingmelting of the winding of the ignition coil 6 and magnetic saturation ofthe transformer.

Limiting of the collector current Ic is realized by a mechanismdescribed below. A sense current Ies from the IGBT 41 flows through asecond resistor 24 in the integrated circuit 3 to generate a voltageacross the second resistor 24 according to the collector current Ic ofthe IGBT 41. This voltage is compared with a voltage Vref1 of a firstreference voltage source 22 by an amplifier 21. A V-I conversion circuit20 outputs a current If1 according to the difference between thecompared values. From this current If1, a second current mirror circuitconstituted by a fourth Pch MOS 13 and a fifth Pch MOS 14 produces anoutput current according its mirror ratio. This output current is outputas a current-limiting signal If2. The current-limiting signal If2 actsin the direction to reduce the current Ig2 from which the gate drivevoltage to the IGBT 41 is generated. As a result, the gate voltage isreduced to inhibit the collector current Ic from increasing. That is,the entire system operates in a negative feedback manner with respect tothe collector current Ic, thereby limiting the collector current Ic to apredetermined constant value.

When the collector current Ic becomes equal to the current limit valueat time t4, the gate voltage to the IGBT 41 is lower and the IGBT 41operates in pentode fashion. That is, while the collector current Ic isflowing, the collector voltage is not sufficiently reduced; Joule lossis being produced in the IGBT 41.

When the operation temperature is increased, the allowable powerdissipation of the IGBT 41 is reduced. Therefore, an overheat protectionfunction to limit Joule loss according to temperature is required forprotection of the IGBT 41. The mechanism of the overheat protectionfunction will be described below.

The cathode of the Schottky barrier diode 43 mounted on thesemiconductor switching device 4 is connected to the reference side of athird current mirror circuit constituted by a sixth Pch MOS 15 and aseventh Pch MOS 16 in the integrated circuit 3. Also, the output currentIs2 from the third current mirror circuit acts in the direction toreduce the current Ig2 from which the gate drive voltage to the IGBT 41is generated, as in the case of the above-described current-limitingfunction.

The reverse saturation current Is through the Schottky barrier diodeincreases abruptly when the temperature exceeds about 170° C., as shownin the temperature characteristic graph in FIG. 3.

Thus, when the operating temperature exceeds about 170° C., thecollector current Ic is reduced by reducing the gate drive voltage bymeans of the Schottky barrier diode 43 and the third current mirrorcircuit constituted by the sixth Pch MOS 15 and the seventh Pch MOS 16.The overheat protection function to limit Joule loss in the IGBT 41 isthus realized.

The above-described mechanism has the effect of reducing the current Ig2from which the gate drive voltage is generated in common with theabove-described current-limiting function. In other words, theabove-described overheat protection function is a function to set thecurrent limit value lower than it is during the normal operation whenthe operating temperature exceeds about 170° C., as shown in FIG. 4.

The overheat protection function in the first embodiment is not topositively shut off the IGBT 41 but only to reduce the collector currentlimit value for the IGBT 41. That is, no shutoff is made at any timeother than the proper ECU 1 timing, and prevention of erroneous ignitionwith the ignition plug 7 is enabled without additionally providing asoft shutoff function.

If the operating temperature continues to increase, the current limitvalue continues to decrease, finally making impossible the supply ofenergy sufficient for causing arc discharge on the ignition plug 7.However, generally speaking, the operating speed of the ECU 1 isextremely high in comparison with rising of the operating temperature.There is, therefore, a sufficient time period from a start of overheatprotection to a point in time at which a misfire actually occurs, and asufficient time margin can be taken in which the ECU 1 can detect amisfire due to overheat protection and take suitable steps.

Second Embodiment

FIG. 5 shows a second embodiment of the igniter power semiconductordevice according to the present invention. In the drawings, componentsequivalent in function to each other are indicated by the same referencecharacters. Description will not be redundantly made for them.

The second embodiment has a feature in that the Schottky barrier diodemounted on the semiconductor switching device 4 in the first embodimentis mounted in the integrated circuit 3. In the igniter powersemiconductor device 5, the semiconductor switching device 4 and theintegrated circuit 3 are disposed close to each other on a sameconductor circuit board. Therefore the thermal coupling between thesecomponents is markedly good. For this reason, the same effect as that inthe case where the temperature sensing element is mounted on thesemiconductor switching device 4 can also be obtained.

It is more desirable that the Schottky barrier diode 25 in theintegrated circuit 3 be mounted at a position close to the semiconductorswitching device 4 in the layout, e.g., in the vicinity of the side ofthe integrated circuit 3 facing the switching device 4.

In the present embodiment, the connection lines and pads for theSchottky barrier diode 43 required in the first embodiment can beremoved; the efficiency of layout patterning of the semiconductorswitching device 4 is improved; and the components can be disposed withimproved area efficiency. Therefore the igniter power semiconductordevice 5 can be implemented in a reduced size at a low cost.

It is possible to positively utilize the state where the Schottkybarrier diode 25 provided as a temperature sensing element is mounted inthe integrated circuit 3. For example, a Schottky barrier diode may beused as the diode used to form the constant-current source 19 in placeof the ordinary PN junction type to adjust a temperature characteristicof the constant-current source to that of the Schottky barrier diode 25provided as a temperature sensing element.

It is possible to make steeper the current limit value reductioncharacteristic at the time of overheat protection as well as thetemperature characteristic of the temperature sensing element by givinga temperature characteristic to the constant-current source 19. Anextremely high degree of characteristic matching between the componentscan be achieved in the same integrated circuit 3. Therefore thetemperature characteristics of the constant-current source 19 and theSchottky barrier diode 25 provided as a temperature sensing element canbe matched to each other with high accuracy.

Third Embodiment

FIG. 6 shows a third embodiment of the igniter power semiconductordevice according to the present invention. In the first and secondembodiments, there is a possibility of failure to obtain the desiredreduction characteristic with respect to the current limit value at thetime of overheat protection due to manufacturing process variation inthe reverse saturation current through the Schottky barrier diode. Thisvariation may be adjusted through an external connection terminal toimprove the yield of the product and to enable the adjustment of thecurrent limit value attenuation sensitivity according to use of theproduct.

In the example of the circuit shown in FIG. 6, three temperature sensingelement selecting circuits S1, S2, and S3 having different outputcurrent values are provided and selection between validity andinvalidity of each temperature sensing element output from the outsideof the igniter power semiconductor device 5 is enabled.

Schottky barrier diodes 25 are respectively incorporated in each of thetemperature sensing element selecting circuits S1 to S3. The sizes ofthe diodes therein are binary-weighted (for example, if the size in S1is 1, the size in S2 is 2 and the size in S3 is 4). Further, eachtemperature sensing element selecting circuit is validated/invalidatedby turning on/off an eighth Pch MOS 26. To turn on/off the eighth PchMOS 26, an external terminal is grounded or opened.

In this way, selection of the Schottkey barrier diode size from theeight sizes: 0 to 7 can be made through a combination of the temperaturesensing elements in the circuits S1 to S3. Selection among thetemperature sensing elements may be set from the outside of the igniterpower semiconductor device 5 as in the present embodiment. If adjustmentis performed only in the manufacturing process, the arrangement may besuch that no external terminals are provided and selection from validityand invalidity of each temperature sensing element is made through theexistence/nonexistence of wire bonding between pads from S1 to S3provided on the integrated circuit 3 and a ground terminal.

Fourth Embodiment

FIG. 7 shows a fourth embodiment of the igniter power semiconductordevice according to the present invention. As described in the firstembodiment, when the overheat protection operation is started inresponse to an increase in operating temperature, it is desirable toperform suitable feedback steps such as notifying the ECU 1 of thesituation, referring to table data of the control signal ON time andreducing the engine output.

In the present embodiment, an overheat protection operation conditionoutput circuit 10 is provided in the integrated circuit 3. A ninth PchMOS 40 is connected to the output side of the third current mirrorcircuit that detects the reverse saturation current Is1 through theSchotkky barrier diode 25 provided as a temperature sensing element, andan output current Is3 from the ninth Pch MOS 40 flows through a thirdresistor 43. The voltage generated across the third resistor 43 and avoltage Vref2 of a second reference voltage source 42 are compared by acomparator 41. An output from the comparator 41 is taken out of theigniter power semiconductor device 5 to be monitored by the ECU 1.

With the igniter power semiconductor device 5 in the fourth embodimentconfigured as described above, the ECU 1 can always grasp whether or notthe overheat protection operation is presently being performed from theoutput of the comparator 41 and can therefore perform suitable feedbacksteps.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2009-278422,filed on Dec. 8, 2009 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A power semiconductor device for an igniter comprising: asemiconductor switching device causing a current to flow through aprimary side of an ignition coil or shutting off the current flowingthrough the primary side of the ignition coil; an integrated circuitdriving and controlling the semiconductor switching device; and atemperature sensing element sensing temperature of the semiconductorswitching device, wherein the integrated circuit including an overheatprotection circuit limiting a current through the semiconductorswitching device to a value lower than a current through thesemiconductor switching device during normal operation, when temperaturesensed by the temperature sensing element is over predeterminedtemperature.
 2. The power semiconductor device for an igniter accordingto claim 1, wherein the temperature sensing element and thesemiconductor switching device are provided on a common substrate. 3.The power semiconductor device for an igniter according to claim 1,wherein the temperature sensing element and the integrated circuit areprovided on a common substrate.
 4. The power semiconductor device for anigniter according to claim 1, wherein the temperature sensing elementincludes a Schottky barrier diode.
 5. The power semiconductor device foran igniter according to claim 1, wherein temperature characteristic ofthe temperature sensing element can be externally adjusted.
 6. The powersemiconductor device for an igniter according to claim 1, furthercomprising an overheat protection operation condition output circuitoutputting a signal indicating that the overheat protection circuit islimiting the current through the semiconductor switching device.